MANET (Mobile Ad Hoc Network) is a type of ad hoc network that can change locations and configure itself, because of moving of nodes. As MANETs are mobile in nature, they use wireless connections to connect various networks without infrastructure or any centralized administration. While the nodes communicate with each other, they assist by forwarding data packets to other nodes in the network. Thus the nodes discover a path to the destination node using routing protocols. Gray hole attack among the different types of attacks possible in a MANET. Gray Hole attack is one type of active attack which tends to drop the packets during transmission the routing from source to destination .In this paper, we simulate gray hole attack Detection and prevention technique using AODV and AODV+PSO. Performance metrics Packet dropped or packet loss, End to End delay and Average throughput, the performance analysis has been done by using simulation tool ns-2 which is the main infrastructure.